Slotted rotor question (RDA)

[mattp]

Hi all, I just got my 'new' Tickford brake set-up for my AU1, which included brand new RDA slotted rotors (which are huge!). The calipers etc need a clean up, but all looks good. Anyway, my question is, what is this strange bit 'missing' from the rotor, looks like it's been machined out so I assume it's meant to be there, but just curious as to it's purpose... It's in the second photo, and it's only one rotor that has it. My old RDA slotted rotors don't have any bit like this on them. (Sorry it's a little blurry)
Thanks in advance.